1. Field of the Invention
The present invention relates to a carbon brush that is used in a rotary electric machine such as a starter for cranking an internal combustion engine.
2. Description of Related Art
A starter including a DC motor is used in an automotive vehicle for cranking an internal combustion engine. A high vibration is imposed on the starter because the starter is directly mounted on the engine. Especially, the starter is subjected to a very high vibration when it is mounted on an engine rotating at a high speed. Therefore, the starter has to be designed to endure a high vibration. Carbon brushes used in the DC motor are also subjected to a high vibration.
A lead wire of the carbon brush is made of twisted copper wires to give a flexibility to the lead wire so that the lead wire is able to follow changes in a length of the carbon brush due to abrasion wear. The lead wire has a margin length between both ends thereof in order to give a certain amount of flexure. When vibration is applied to the lead wire, the lead wire vibrates by the amount of the flexure. There is a possibility that the lead wire is broken due to fatigue caused by such vibration. In particular, the lead wire used in the starter has a limited length for giving flexibility because of a space limitation. Since the starter is exposed to a high vibration, the lead wire vibrates up to an amount of the flexure created by the margin length. This vibration may result in cutting-off of the lead wire.
To cope with this problem, JP-B2-62-32698 proposes an improved lead wire having a high toughness. To improve the toughness of the lead wire, soft copper wire elements and stainless steel wire elements (or piano wire elements) are twisted together to form the lead wire. However, there is an inverse relation between toughness and flexibility. That is, flexibility of the lead wire has to be sacrificed by improving the toughness. If the flexibility of the lead wire decreases, a force pressing the carbon brush against a commutator decreases, resulting in a poor contact between the carbon brush and the commutator.